1. Field
The present disclosure relates generally to navigation and, in particular, to using global positioning system (GPS) signals for navigation. Still more particularly, the present disclosure relates to a method and apparatus for reducing the effects of interference when using global positioning satellite signals for navigation.
2. Background
Satellite systems are used to provide position and time information that may be received almost anywhere on Earth where an unobstructed line of sight is present to a sufficient number of satellites transmitting these signals. These types of satellite systems are also referred to as global navigation satellite systems (GNSS). A global positioning system (GPS) is a particular example of a global navigation satellite system.
Global positioning system receivers normally identify their position using these signals. The position of a receiver may be identified by calculating the relative times of arrivals of signals transmitted from these signals. These signals include messages that contain the position of the satellite transmitting the message and clock timing information. The clock timing information is used to identify a time of when the message was sent.
The use of receivers that identify positions based on global positioning system signals from satellites has become widespread. These types of receivers can be found in various objects, such as cars, trucks, ships, aircraft, and other types of platforms.
Interference, however, may occur with receivers receiving the signals. The interference may be caused by various sources. These sources may include, for example, objects obstructing or blocking the signals, background radio frequency signals, intentional transmission of radio frequency signals using frequencies of global positioning system satellites, and other sources.
The interference may be such that signals from these satellites may be blocked and/or degraded such that the receiver is unable to identify the position of the receiver using the signals. For example, when the receiver is in aircraft, dependence on a global positioning system signal from satellites may make navigation difficult and less precise if interference reduces the ability of the receiver to obtain a signal with a sufficient quality to identify the position of the aircraft that is as precise as desired.
As a result, aircraft may include other types of positioning systems, such as an inertial navigation unit to aid in instances when global positioning system signals are unavailable or do not have a desired quality. The other systems may not have the accuracy desired, increase the cost of the aircraft, and/or may increase the weight of the aircraft.
Further, using receivers with increased sensitivity to detect and use global positioning system signals may be a solution that works in some cases. Using receivers with increased sensitivity, however, may be more expensive and may increase the weight and/or size of the system in a manner that is undesirable.
Therefore, it would be advantageous to have a method and apparatus that takes into account at least some of the issues discussed above, as well as possibly other issues.